1. Field of the Invention
The present invention relates to a micro-displacement detector device and, more particularly, a device for detecting the displacement of a probe, which device is used by the scanning probe microscope such as the scanning tunnel microscope (which will be hereinafter referred to as STM) and the atomic force microscope (which will be hereinafter referred to as AFM).
2. Description of the Related Art
In the case of the scanning probe microscope such as STM and AFM, the displacement of the probe relative to a sample is indirectly measured by detecting tunnel current flowing between the sample and the probe or atomic force acting between them.
These STM and AFM employ mainly a piezo-actuator to slightly move the probe. The piezo-actuator includes a piezo-element body and plural drive electrodes arranged round the piezo-element body. When the piezo-element body is expanded or contracted by voltage applied to the drive electrodes, the probe fixed to the piezo-actuator is moved slightly or on micro-basis. The position of the probe (or the amount of expansion and contraction of the piezo-actuator) can be therefore obtained in this case from the voltage applied to the drive electrodes.
It is however difficult to accurately determine the position of the probe from the voltage applied to the drive electrodes. One of the reasons resides in that the piezo-actuator moves not linearly but with hysteresis by the voltage applied to the drive electrodes, as shown in FIG. 7. In short, the piezo-actuator cannot return accurately to its original position even when voltage is stopped after it is applied to the drive electrodes. Another reason is that the piezo-actuator has creep phenomenon, when voltage is applied to the drive electrodes for a long time, therefore, the relation between the voltage applied and the amount of expansion and contraction of the piezo-actuator becomes out of order. A further reason resides in that when the probe comes quite near the surface of a sample, atomic force acts between the probe and the sample to separate the probe from the surface of the sample. As the result, the proportional relation between the voltage applied and the amount of expansion and contraction of the piezo-actuator is broken.
As described above, it is difficult to accurately measure the position (or displacement) of the probe on the basis of voltage applied to the drive electrodes for the piezo-actuator. It is therefore difficult for this piezo-actuator to measure micro-displacements on atomic basis.